The present invention relates to pink borosilicate glasses, a method for their manufacture, and the articles formed from such glasses.
Borosilicate glasses, such as Pyrex.RTM. glass, have been known for a long time and are widely used for the manufacture of cooking articles that are oven-resistant. Indeed, their low coefficient of expansion gives them a good resistance to thermal shock, which is improved even further by the thermal tempering that is generally applied to them. The borosilicate glasses used for cooking are for the most part manufactured without the addition of any coloring agent, and therefore have a slightly yellow or yellow-green color due to the inevitable iron impurities. Certain borosilicate glasses are colored light blue by a few ppm of cobalt. The appearance of articles of colored borosilicate glass is recent. Up to the present it has involved only brown glasses such as those which are described in U.S. Pat. No. 4,116,704.
Different colors have been tested during consumer polls and it appeared that the pink color received many votes.
It is known that the addition of a manganese oxide, such as MnO.sub.2, to certain glass batches imparts to them a pink color which can be attributed to the presence of Mn.sup.3+ ions in the glass. However, it is also known, according to FR A 547,091 filed on Feb. 11, 1922, that in the case of borosilicate glasses, the manganese dioxide does not produce a pink color, but instead a color ranging from yellow to maroon. To the knowledge of the applicant, this teaching has not been contradicted up to the present, even though it is older than 70 years.
Other coloring agents have indeed been proposed, such as selenium, gold, or erbium oxide, in order to color borosilicate glasses pink, but these coloring agents, either because of cost or because of difficult implementation, are ill-suited for the large scale production of borosilicate glass articles, such as Pyrex.RTM., of a pink coloring.
There is currently the need for a pink-colored borosilicate glass that is not expensive and is easy to manufacture.